One can usually gain a few HP by milling the head. Always first want to measure valve to head clearance. I milled the head on my GMC 302 .060" with plenty of clearance, GMC 503 .090" with plenty of clearance(similar engines.)

If not want to waste a head gasket in the process, roll some pieces of modeling clay small enough to fit loosely in spark plug hole, one could wrap the clay around a piece of welding rod for increased rigidity, slip it in the spark plug hole, being careful to find the valves. Turn the engine over several times, by hand, to get your clearance. Of course might want to make sure valves are in spec. first.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.